Bug: startAt highlighting being done everywhere instead of just specific y/m/d

[Vealor]

Problem:

The grey highlighting for startAt is being placed for every month and every year under certain conditions.

Steps to reproduce:

1. Include [startAt]="getFirstDate()" or similar in sat-datepicker line such as <sat-datepicker #pickerFilterRange [startAt]="getFirstDate()" [rangeMode]="true">
2. Notice that grey highlighting is on the given startAt date.
3. Select Year at top and select a random Year.
4. Select a random Month.
5. Grey highlighting associated with the day of startAt is also being applied to the same day in this month & year despite not being the month and year for startAt.

Note: This is reproducible in the DEMO with extra features examples in the README

[LeParadoxHD]

I believe I wrote an issue about the same. #141